Phosphonic acid compounds, and their salts, are well known water treatment chemicals. Various phosphonates are used as sequestering agents, scale inhibitors, corrosion inhibitors and additives for use generally in water treatment applications, such as in circulating water systems. Particularly useful are polyphosphonates. Among the most common commercially available polyphosphonates are amino (methylenephosphonic acids) such as amino tri(methylenephosphonic acid), ethylenediamine tetra (methylenephosphonic acid), and others; and hydroxy-phosphonates such ethane-1-hydroxy-1,1-diphosphonic acid. Many additional phosphonates are known in the literature, including unsubstituted alkane diphosphonates, such as 1,2-ethane diphosphonates, whose use for corrosion inhibition is disclosed in U.S. Pat. No. 4,209,487, which is incorporated herein by reference.
Phosphonate condensation polymers have been produced from the reaction of a phosphonic acid group with an alcohol or with an amine having a reactive hydrogen. A polymeric product can be produced by reacting a polyphosphonate or an anhydride of a polyphosphonate with a polyhydric alcohol as is described in U.S. Pat. No. 3,395,113 and U.S. Pat. No. 3,470,112, or by reacting a hydroxyphosphonate such as ethane-1-hydroxy-1,1-diphosphonic acid, as is described in U.S. Pat. No. 3,621,081, in each case forming a polyester. A polymeric product can also be formed by reacting a polyphosphonate anhydride with a polyamine as is described in U.S. Pat. No. 3,645,919. Formation of these polymeric products requires the presence of a reactive hydrogen on either an alcohol or an amine group.
Polymeric bisphosphinic acid anhydrides are described in U.S. Pat. No. 4,196,141, although the degree of polymerization is not specified. These bisphosphinic acid anhydrides are prepared by reacting salts or esters of the bisphosphinates with inorganic acid chlorides, phosgene, oxalyl chloride, the corresponding bromine compounds, or mixtures thereof, at a molar ratio of about 1:1. These bisphosphinate anhydrides are said to be useful as comonomers in the preparation of plastics, particularly polyesters, and for preparation of flame proofing agents. There is no discussion of water treatment uses.
Each of the phosphonates used in water treatment has its individual drawbacks. For instance, aminophosphonates and hydroxyphosphonates are efficient inhibitors of calcium carbonate scale, but are unstable in the presence of oxidants, such as chlorine. Alkylene diphosphonates, such as ethane-1,2-diphosphonic acid, are stable to chlorine, but are relatively inefficient calcium scale inhibitors.
Phosphates are generallly better corrosion inhibitors than phosphonates, when used in circulating water systems. However, the formation of calcium phosphate scale can be a problem, because of the presence of calcium ions. This scale formation removes some phosphate from solution, reducing the amount available for corrosion inhibition, and also forms undesirable deposits.
A phosphonate with an improved mix of scale inhibition, corrosion inhibition, and chlorine stability properties would be a significant advance in the art and is an object of this invention.